Distributed Weighted Fair Queuing in 802.11 Wireless LAN Albert Banchs, Xavier P´erez NEC Europe Ltd., Network Laboratories Heidelberg, Germany Abstract— With Weighted Fair Queuing, the link’s bandwidth Specifically, the station computes a random value in the is distributed among competing flows proportionally to their range of 0 to the so-called Contention Window (CW). A backoff weights. In this paper we propose an extension of the DCF func- Ì = time interval is computed usingthis random value: back Óf f tion of IEEE 802.11 to provide weighted fair queuing in Wireless Ì Ê aÒd´¼;Cϵ £ Ì ×ÐÓØ LAN. Simulation results show that the proposed scheme is able ×ÐÓØ ,where is the slot time. This backoff to provide the desired bandwidth distribution independent of the interval is then used to initialize the backoff timer. This timer flows aggressiveness and their willingness to transmit. Backwards is decreased only when the medium is idle. The timer is frozen compatibility is provided such that legacy IEEE 802.11 terminals when another station is detected as transmitting. Each time the receive a bandwidth corresponding to the default weight. medium becomes idle for a period longer than DIFS, the back- Index Terms— Wireless LAN, IEEE 802.11, MAC, Weighted off timer is periodically decremented, once every slot-time. Fair Queuing, Bandwidth Allocation As soon as the backoff timer expires, the station accesses the medium. A collision occurs when two or more stations start I. INTRODUCTION transmission simultaneously in the same slot. An acknowledg- ment is used to notify the sendingstations that the transmitted Much research has been performed on ”weighted fair queu- frame has been successfully received. ing” algorithms for achieving the desired bandwidth allocation If an acknowledgment is not received, the station assumes on a wired link [1], [2], [3], [4], [5], [6], [7]. With weighted fair that the transmitted frame was not received successfully and queuing, the bandwidth received by a flow in a shared link is in schedules a retransmission reenteringthe backoff process. To proportion to the flow’s weight. reduce the probability of collisions, after each unsuccessful Since Wireless LANs may be considered as just another tech- transmission attempt, the CW is doubled until a predefined nology in the communications path, it is desirable that the archi- CÏ maximum ( ÑaÜ ) is reached. After a (successful or un- tecture for bandwidth allocation follows the same principles in successful) frame transmission, if the station still has frames the wireless network as in the wireline Internet, assuringcom- queued for transmission, it must execute a new backoff process. patibility amongthe wireless and the wireline parts. One problem concerningthe carrier sensingof the DCF The challenge in Wireless LAN is that we do not have all mode is the so-called hidden station problem. A station, that packets in a centralized queue, like in wired links, but we have may not be within receivingrangeof a sendingstation and thus them distributed in the wireless hosts. Therefore, we need to senses the medium idle, may however well be within sending design a new MAC mechanism for Wireless LAN capable of range of the receiver of that ongoing communication and may providingthe desired scheduling. thus cause a collision there, if it starts transmittingitself. To In this paper we propose a fully distributed approach, Dis- deal with the hidden station problem, the DCF MAC protocol tributed Weighted Fair Queuing (DWFQ), that extends the DCF can use the Request To Send (RTS) / Clear To Send (CTS) mode of the 802.11 MAC protocol to distribute the bandwidth mechanism. When the RTS/CTS mechanism is applied, the of the wireless network amongthe different flows proportion- winningstation does not send data packets rightaway but sends ally to their weights. a RTS packet to the receivingstation, that responds with a CTS The rest of the paper is structured as follows. Section II re- packet. If a station captures a RTS packet from another sta- calls the basics of the IEEE 802.11 standard. In Section III tion and it is not the destination of the RTS packet it reads the we describe the DWFQ architecture for providingweighted fair intended transmission duration from the RTS packet and stays queuingin Wireless LAN. Simulation results are presented in silent for that time. The same happens if only a CTS packet the followingsection. The paper closes with an overview on is received i.e. by a station outside the transmission range of related work and the conclusions (Sections V and VI). the sender but within the range of the receiver. This guarantees that all stations within the range of either sender or receiver II. THE IEEE 802.11 MAC LAYER have knowledge of the transmission as well as of its duration. The basic IEEE 802.11 Medium Access mechanism is called Besides reducingthe collision probability in the case of a hid- Distributed Coordination Function (DCF) and is based on den station, another effect of the RTS/CTS mechanism is that the Carrier Sense Multiple Access with Collision Avoidance it increases bandwidth efficiency since, if collisions occur, they (CSMA/CA) protocol. In the DCF mode, a station must sense do not occur with longdata packets but with the relative small the medium before initiatingthe transmission of a packet. If control packets. the medium is sensed like beingidle for a time interval greater The second access mechanism specified in the IEEE stan- than the Distributed Inter Frame Space (DIFS), then the station dard is built on top of DCF and it is called Point Coordina- transmits the packet. Otherwise, the transmission is deferred tion Function (PCF). It is a centralized mechanism, where one and a backoff process is started. central coordinator polls stations and allows them undisturbed, 0-7803-7400-2/02/$17.00 (C) 2002 IEEE Ö i contention free access to the channel. In contention free ac- where i is the estimated bandwidth experienced by flow and Ï Ö i cess mechanism, collisions do not occur since the access to the i is its weight. The estimated throughput, , is updated every channel is controlled by one entity. time a new packet is transmitted: In order to separate the different types of packets, different Ð i Ø =à ÓÐd ÒeÛ Ø =à i i · e Ö =´½ e µ Ö (3) i levels of access priority are implemented by defining3 inter- i Ø frame spaces (IFS) of different length. They define the minimal i Ð Ø i time, that a station has to let pass after the end of a packet, where i and are the length and inter-arrival time of the trans- before it may start transmittinga certain type of packet itself. mitted packet, and à is a constant. Followingthe rationale =½¼¼Ñ× After SIFS (Short IFS), the shortest interframe space, only ac- discussed in [7], in this paper we set à . Ä knowledgments, CTS and data frames in response to poll by With the above definition of the label i , the resource dis- the PCF may be sent. After PIFS (PCF-IFS), any frames from tribution expressed in Equation 1 can be achieved by imposing Ä the contention free period may be sent in PCF-mode, after the condition that the label i should have the same value for DIFS (DCF-IFS), the longest of the three interframe spaces, all the flows: Ä Ä 8i all frames in DCF-mode may be sent. This use of IFS allows = i (4) the most important frames to be sent without additional delay Note that the actual value of Ä can vary in time (depending and without havingto compete for access with lower priority on the number of flows for example). frames. It allows the prioritized access to the medium for the Equation 4 is fulfilled by usingthe followingalgorithm: hav- PCF mode over the contention mode frames in the DCF mode. Ä ingcalculated its own label i , each station includes its label in the header of the packets it sends. For each observed packet, if Ä ISTRIBUTED EIGHTED AIR UEUING Ä i III. D W F Q the i in the packet’s header is smaller than the of the sta- With weighted fair queuing, the bandwidth experienced by a tion, the station increases its CW by a small amount, while in the opposite case it decreases its CW by a small amount. In this i Ö flow , i , is proportional to the weight that this flow has been Ä Ä way, the i of all flows tend towards a common value, . Ï assigned, i : The above explanation describes the basics of the algorithm. Ö Ö j i However, in the adjustment of the CW, there are additional as- 8i; 8j = (1) Ï Ï i j pects that have to be taken into account: ¯ We do not want the CW to increase above the values de- In the DCF approach, the bandwidth received by a flow de- fined by the 802.11 standard; as argued above, for the pends on its CW: the smaller the CW, the higher the throughput. backward compatibility reasons the basic service (with a In DWFQ, weighted fair queuing in Wireless LAN is supported weight equal to 1) uses the CWs defined in the 802.11 by the DCF function of the current standard with minor changes standard, and any higher weight should receive a ”better in the computation of the CW in order to give to each flow a than average” kind of treatment and therefore the values bandwidth proportional to its weight. of the CW should be lower. Accordingto the above explanation, terminals conformingto ¯ If the low sendingrate of the application is the reason for the IEEE 802.11 standard and DWFQ terminals compete with transmittingbelow the desired rate, then the CW should each other with different CW.